1. Field of the Invention
The present invention relates to a navigation apparatus and a navigation method, and more particularly, it relates to a display device of an on-vehicle navigation apparatus.
2. Description of the Related Art
There is a self-sustaining type navigation apparatus as a measurement apparatus for various movable bodies, such as cars, airplanes, vessels, and so on. This self-sustaining type navigation apparatus obtains a two dimensional displacement (vector) of the movable body from azimuth data obtained by an azimuth sensor and speed data obtained by a speed sensor, and then, integrates thus obtained two dimensional displacement with respect to a standard point, so as to obtain the present position. For example, in the case of a car, the navigation apparatus obtains the present position (data) by integrating the integrated travelling distance and the integrated azimuth obtained from a travelling distance sensor and an azimuth sensor respectively, with respect to the standard point. More concretely, for example, the number of rotations of a drive shaft, and the number of pulses generated by the number-of-rotation sensor attached to the drive shaft, are made in correspondence with each other beforehand. The integrated travelling distance is obtained by multiplying a distance correction coefficient to the distance computed from the total number of pulses generated during the time from the standard position to the present position. The integrated azimuth is obtained by integrating the azimuth obtained by the earth magnetism sensor.
There is a GPS (Global Positioning System) navigation apparatus is a measurement apparatus using the artificial satellites. This GPS navigation apparatus normally receives electric waves from three or more GPS satellites, and then, obtains pseudo-distance data including a time offset of the receiver between each GPS satellite and a receiving point (i.e. self-position), position data of each GPS satellite, and the present position (data) of the receiving point.
As an application mode of these measurement apparatuses to an actual GPS navigation apparatus, there is a simple apparatus which indicates latitude and longitude of the present position numerically, and there is also an advanced apparatus which displays various-kinds of data such as a present position, distance to a destination, travelling velocity, etc., on the map displayed on the picture plane of a CRT (Cathode Ray Tube).
The navigation apparatus, which displays various data on the CRT picture plane, reads out map data, which includes the obtained present position, from a memory medium, such as a CD-ROM (Compact Disk--Read Only Memory). Then, the navigation apparatus performs map matching by use of the read map data and the obtained present position data, generates picture-plane data, and displays the self present position, the advancing direction etc. on the map picture plane of the CRT. By this display image, a user can grasp the self present position in relation to the map.
Map matching is explained here.
Digitized map data includes information as for the azimuth and distance as the inclination and length, respectively, of a line segment. The technique of using this information for the azimuth and distance to guess the exact present position, is called map matching. Even if the detected azimuth shifts slightly due to the error of the sensor, if a possibility that the present position is on the road on which the vehicle has been travelling until now, is judged to be high by this map matching, for example, the present position is corrected onto the road, so that an appropriate display without incongruity can be performed.
In order to use or check the travelling history of the vehicle, the above-mentioned navigation apparatus stores the present position data used for the display for every predetermined time interval to a memory, and, by a predetermined operation, connects the present positions, which are stored in the memory and are changing every moment, to display a travelling locus (travelling history).
Apart from that, when performing map matching in a navigation apparatus, two or more present position candidacy data are obtained, and, among these, the present position candidacy data which is judged to be the most probable one i.e. the most reliable one among these, is used as the present position data. Thus, the present position is displayed at real time. The present position data for displaying the position at real time is also used as the present position data for displaying the travelling locus.
Therefore, even when the present position data, which has been judged to be the most probable one at the time of displaying the present position, is judged to exist on a road different from a road on which the vehicle is estimated to have actually travelled by considering the travelling condition afterward, this erroneous present position data is used for displaying the travelling locus without being corrected.
Consequently, there is a problem that a discontinuous portion is generated in the display of the travelling locus, which should be in fact continuous.